The Concept of 'Separation' in the Writings of Irenaeus [Part One]

The importance of 'unity' and the evil of 'separation' is a fundamental concept in Irenaeus.  If we turn to the Apostolic Preaching we read of a core 'antithesis' in his theology addressed to a certain 'Marcian':

We send you as it were a manual of essentials, that by little you may attain to much, learning in short space all the members of the body of the truth, and receiving in brief the demonstration of the things of God. So shall it be fruitful to your own salvation, and you shall put to shame all who inculcate falsehood, and bring with all confidence our sound and pure teaching to everyone who desires to understand it. For one is the way leading upwards for all who see, lightened with heavenly light: but many and dark and contrary are the ways of them that see not. This way leads to the kingdom of heaven, uniting man to God: but those ways bring down to death, separating man from God. Wherefore it is needful for you and for all who care for their own salvation to make your course unswerving, firm and sure by means of faith, that you falter not, nor be retarded and detained in material desires, nor turn aside and wander from the right. God.

So it is said later in the 'instruction manual' that union with Jesus - to overcome his separation - his key to salvation:

And there is none other name of the Lord given under heaven whereby men are saved, save that of God, which is Jesus Christ the Son of God, to which also the demons are subject and evil spirits and all apostate energies, by the invocation of the name of Jesus Christ, crucified under Pontius Pilate.  He is separated and withdrawn from among men, and there is a separation and division among mankind; and wheresoever any of those who believe on Him shall invoke and call upon Him and do His will, He is near and present, fulfilling the requests of those who with pure hearts call upon Him. Whereby receiving salvation, we continually give thanks to God, who by His great, inscrutable and unsearchable wisdom delivered us, and proclaimed the salvation from heaven ---- to wit, the visible coming of our Lord, that is, His living as man ----to which we by ourselves could not attain.

While none of this specifically relates to the issues we were dealing with earlier - i.e. the separation of the godhead in heaven - the context for this instruction is again reinforced to be developed against a heretical community which promotes 'separation.'